This invention relates to signal processing, and more particularly, to signal processing techniques for use in Digital Audio Broadcasting (DAB) systems.
Digital Audio Broadcasting is a medium for providing digital-quality audio, superior to existing analog broadcasting formats. Both AM and FM DAB signals can be transmitted in a hybrid format where the digitally modulated signal coexists with the currently broadcast analog AM or FM signal, or in an all-digital format without an analog signal. In-band-on-channel (IBOC) DAB systems require no new spectral allocations because each DAB signal is simultaneously transmitted within the same spectral mask of an existing AM or FM channel allocation. IBOC systems promote economy of spectrum while enabling broadcasters to supply digital quality audio to their present base of listeners. Several IBOC DAB approaches have been suggested.
FM DAB systems have been the subject of several United States patents including U.S. Pat. Nos. 5,465,396; 5,315,583; 5,278,844 and 5,278,826. More recently, a proposed FM IBOC DAB signal places orthogonal frequency division multiplexed (OFDM) sub-carriers in the region from about 129 kHz to 199 kHz away from the FM center frequency, both above and below the spectrum occupied by an analog modulated host FM carrier. Some IBOC options (e.g., All-Digital option) permit subcarriers starting as close as 100 kHz away from the center frequency.
The digital portion of the DAB signal is subject to interference, for example, by first-adjacent FM signals or by host signals in Hybrid IBOC DAB systems. Signal processing techniques are required to separate out the signals of interest in the presence of the interferers.
One FM extraction technique called COLT (COntinuous Look Through) can be used to extract a narrowband signal from beneath a wideband FM signal. This technique is described in U.S. Pat. Nos. 5,263,191; 5,428,834; and 5,355,533. The method described in those patents uses, in effect, a notch filter which tracks and suppresses the FM instantaneous frequency of an interfering signal.
FM IBOC DAB signals are subject to various fading conditions and bandwidth properties which limit that effectiveness of prior art COLT techniques. The is a need for a signal extraction technique that is effective for in-band on-channel digital audio broadcast signals under fading conditions.